The present invention relates generally to photomultiplier tubes and more particularly to a photomultiplier tube constructed for use in time resolving picosecond luminescent events and/or picosecond voltage pulse.
A photomultiplier tube is a well known type of photosensitive device that is commonly used in time resolving optical signals.
Basically, a photomultiplier tube comprises a photocathode, an electron multiplier and an anode, all disposed in an evacuated glass housing, with potential differences set up between the electrodes and the electron multiplier to cause photoelectrons emitted by the photocathode to pass through the electron multiplier and or to the anode.
When light strikes the photocathode, photoelectons are emitted into the vacuum in proportion to the intensity of the light. These photoelectrons are multiplied by the electron multiplier and then collected by the anode as an output signal.
Because of the electron multiplication, photomultiplier tubes are uniquely sensitive among photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. Photomultiplier tubes also feature relatively fast time response and low noise.
The photocathode in a photomultiplier tube is generally arranged in either a side-on or a head-on configuration. In the side-on type configuration the photocathode receives incident light through the side of the glass housing, while, in the head-on type, light is received through the end of the glass housing. In general, the side-on type photomultiplier tube is widely used for spectrophotometers and general photometric systems. Most of the side-on types employ an opaque photocathode (reflection-mode photocathode) and a circular-cage structure electron multiplier which has good sensitivity and high amplification at relatively low supply voltage.
The head-on type photomultiplier tube has a semitransparent photocathode (transmission-mode photocathode) deposited upon the inner surface of the entrance window while in the side-on type the photocathode is a separate structure. Because the head-on type provides better uniformity and lower noise, it is frequently used in scintillation detection and photon counting applications.
The electron multiplier in a photomultiplier tube is usually either a series of electrodes, called dynodes, or a microchannel plate. As is known, a microchannel plate (MCP) is a form of secondary electron multiplier consisting of an array of millions of glass capillaries (channels) having an internal diameter ranging from 10 .mu.m to 20 .mu.m fused into the form of a thin disk less than 1 mm thick. The inside wall of each channel is coated with a secondary electron emissive material having a proper resistance and both ends of the channel are covered with a metal thin film which acts as electrodes, thus each channel becomes an independent secondary electron multiplier.
When a voltage is applied between the both sides of an MCP, an electric field is generated in the direction of the channel axis. When an electron hits the entrance wall of the channel, secondary electrons are produced. These secondary electrons are accelerated by the electric field, and travel along the parabolic trajectories determined by their initial velocity. Then they strike the opposite wall and produce other secondary electrons. This process is repeated many times along the channel, and, as a result, the electron current increases exponentially towards the output end of the channel.
The photocathode in a head-on type photomultiplier tube is generally circularly shaped and in a side-on photomultiplier tube is usually in the shape of a portion of a cylinder.
In U.S. Pat. No. 3,885,178 there is disclosed a photomultiplier tube (PMT) which converts a received light signal to an output electrical signal of substantially greater intensity by employing a photocathode to convert incident light to free electrons, a plural dynode accelerating structure for effectively multiplying the free electrons, and an impact ionization diode (IID) for further multiplying and collecting the free electrons to provide a corresponding electrical output signal. The PMT can be an electrostatic device, in which the photocathode and the dynodes are mounted in opposed staggered positions, or a static crossed field device, in which the photocathode and the dynodes all are mounted opposite an accelerating rail and a magnetic field is provided to urge the electrons laterally along the tube. The IID's junction is reverse biased and the entire didode is maintained at a substantially higher potential than the last dynode. The PMT can be gain controlled or turned off without affecting dynode potentials by controlling the IID's potential. Due to the gain provided by the IID, dynode current can be reduced greatly, thereby to increase substantially the tube's life without affecting it's overall gain.
One of the limitations of photomultiplier tubes is that although they have a relatively fast time response they are not capable of time resolving events in the picosecond time regime. On the other hand, a device that does have the capability of time resolving events in the picosecond time regime is the streak camera.
Streak cameras are about fifteen years old in the art and have been used, hitherto, to directly measure the time dynamics of luminous events, that is to time resolve a light signal. A typical streak camera includes an entrance slit which is usually rectangular, a streak camera tube, input relay optics for imaging the entrance slit onto the streak camera tube, appropriate sweep generating electronics and output-relay optics for imaging the streak image formed at the output end of the streak camera tube onto an external focal plane. The image at the external local plane is then either photographed by a conventional still camera or a television camera. The streak camera tube generally includes a photocathode screen, an accelerating mesh, sweeping electrodes and an phosphor screen. The streak camera tube may also include a microchannel plate. Light incident on the entrance of the streak camera is converted into a streak image which is formed on the phosphor screen with the intensity of the streak image from the start of the streak to the end of the streak corresponding to the intensity of the light incident thereon during the time window of the streak. The time during which the electrons are swept to form the streak image is controlled by a sweep generator which supplies a very fast sweep signal to the sweeping electrodes. The input optics of the streak camera, in the past, has been a single lens.
In U.S. Pat. No. 4,659,921 a light detector which can be gated on and off over an ultrashort time window, such as in picoseconds or femtoseconds, is disclosed. The light detector includes, in one embodiment, an input slit for receiving a light signal, relay optics, a sweep generator and a tubular housing, the tubular housing having therein a photocathode, an accelerating mesh, a pair of sweeping electrodes, a microchannel plate, a variable aperture and a dynode chain. Light received at the inputslit is imaged by the relay optics onto the photocathode. Electrons emitted by the photocathode are conducted by the accelerating mesh to the sweeping electrodes where they are swept transversely across the tubular housing at a rate defined by the sweep generator over an angular distance defined by the sweeping electrodes, in a similar manner as in a streak camera. Swept electrons strike the microchanel plate where electron multiplication is accomplished. Exiting electrons which pass through the variable aperture and which strike the first dynode (cathode) in the dynode chain are further multipled and outputted from the last dynode anode in the dynode chain as an analog electrical signal, the analog electrical signal corresponding to the intensity of the light signal during the time window over which swept electrons are picked up by the first dynode. In another embodiment of the invention all of the dynodes in the chain except for the last dynode are replaced by a second microchannel plate.
In U.S. Pat. No. 4,467,189 a framing tube is disclosed which includes a cylindrical airtight vacuum tube, a shutter plate, and a ramp generator. The container has a photocathode at one end thereof and a fluorescent screen at the other end thereof which is opposite to the photocathode. The shutter plate is disposed between and parallel to the surface of the photocathode and fluorescent screen and has a multiplicity of through holes perforated perpendicular to its surface. The shutter plate also carries at least three electrodes that are disposed perpendicular to the axis of the through holes and spaced parallel to each other. The electrodes divide the surface of the shutter plate into a plurality of sections. The ramp generator is connected to the electrodes. The ramp voltage generated changes in such a manner as to reverse its polarity, producing a time lag between the individual electrode. Developing an electric field across the axis of the through holes in the shutter screen, the ramp volage controls the passage of the electron beams from the photocathode through the through holes. A framing camera includes the above-described framing tube and an optical system. The optical system includes a semitransparent mirror that breaks up the light from the object under observation into a plurality of light components and a focussing lens disposed in the path through which each of the light components travels. Each of the light components correspond to each of the sections on the shutter plate. The images of a rapidly changing object are produced, at extremely short time intervals, on different parts of the fluorescent screen.
It is an object of this invention to provide a new and improved photomultiplier tube.
It is another object of this invention to provide a photomultiplier tube that can be used in time resolving picosecond luminescent events and/or picosecond voltage pulses.
It is still another object of this invention to provide a new type of photocathode for a photomultiplier tube.